Periodic determination of pacemaker capture thresholds is important to ensure appropriate pacemaker function. A recent U.S. patent issued to Medtronic, U.S. Pat. No. 5,601,615, describes a system for detecting capture and measuring and finding stimulation thresholds for pacing. Accordingly it is incorporated by this reference in its entirety as it describes the kind of pacemaker systems that could benefit from the invention described in this instant document.
It should be recognized that numerous other capture detection methods and apparatae are described in the patent art and literature and that the teaching of this document is not limited exclusively to use in such systems are described in U.S. Pat. No. 5,601,615. However it is clear that atrial capture detection and threshold measurement is not a fully developed art and so this invention can be applied to any electrical stimulation device that is concerned with atrial capture detection and threshold detection (thresholding) used for such devices.
In patients having dual-chamber pacemakers, assessment of atrial capture thresholds often presents a unique challenge, particularly when confronted by either 1) a noisy or poor quality electrocardiogram (ECG), 2) a poorly visible isoelectric or fractionated (non discrete) P-wave, and/or 3) a concealed P-wave during high rate atrioventricular (AV) pacing. Present techniques for atrial capture detection or thresholding in such circumstances are inadequate.
To be more specific, according to current accepted medical practice, assessment of atrial capture thresholds in dual-chamber pacemaker patients can be an oft-difficult and potentially unnerving task. In many instances, electrocardiographic visualization of atrial capture may be obscured by a noisy or poor quality ECG(electrocardiogram), a poorly visible or fractionated (non-discrete) P-wave, or concealment of the evoked P-wave in the T-wave of the preceding ventricular depolarization (This can easily be seen with reference to FIGS. 6a-c. These Figs. illustrate electrocardiograms that may be taken with surface electrodes and illustrate the obfuscation of atrial capture by a)ECG noise, b)sub optimal ECG vector, and c)concealment by preceding paced QRST complexes, respectively).
Under such circumstances, present methods for improving atrial capture assessment in clinical practice include replacing the programmer ECG electrodes, repositioning the programmer ECG electrodes, and/or resizing the ECG display. In many instances, connecting the patient to a different ECG monitor is the best available solution, but even this is not always adequate. If the attending physician is comfortable with alternatives involving reprogramming the pacemaker to assist in determining atrial thresholds and capture, the physician may additionally respond to atrial capture uncertainty by extending the paced AV interval or changing the test mode to AAI. Manual triggering of hand/eye timed pacing pulses through a PSA (pacing system analyzer)is not practicable.
In patients having sinus bradycardia and intact A-V conduction, the task of measuring atrial capture thresholds can be greatly facilitated using deductive reasoning. An assumption of atrial capture can be inferred by observing the ventricular response to atrial overdrive pacing. Taking this assumption, direct observation of atrial capture on the electrocardiogram (ECG) can be considered redundant. Capture is apparent in FIG. 7 by observing the ventricular heart rate response to overdrive AAI (or AOO) pacing. A sudden decrease in ventricular heart rate to the underlying sinus rate indicates loss of atrial capture. This indirect assessment of atrial capture threshold is made by observing a loss of atrial capture (LOAC) by a sudden drop in the ventricular rate, here illustrated as a missing QRST complex at the LOAC indicator point in FIG. 7.
In patients with complete heart block and a slow ventricular escape rhythm, AAI pacing can facilitate visualization of atrial capture determination by extending ECG isoelectric time. This is illustrated In FIG. 8 where the ventricular rhythm (temporal linear spacing of QRST complexes) is unrelated to the AP (atrial pace) markers on the electrogram.
Caution is important in applying this AAI based technique, since pacemaker patients are prone to asystole and often have significant symptoms upon sudden cessation of ventricular pacing. Brady induced ventricular tachyarrhythmias can also be a concern in some patients, i.e. long QT syndrome). Because of the risks involved, less experienced clinicians are often reluctant to attempt to measure atrial thresholds using the AAI mode, and often forego this opportunity for threshold adjustment altogether, preferring instead to merely set the atrial stimulation pulse at or near the maximum energy level and thus shorten the potential life of the pacemaker because of its limited battery capacity.
An alternate method to indirectly assess atrial capture while providing ventricular support involves monitoring the pacemaker's telemetered intracardiac atrial electrogram (AEGM) signals and/or marker channel. This method is described by Feuer J M, Florio J, and Shandling A H in an article titled: "Alternate methods for the determination of atrial capture threshold utilizing the telemetered intracardiac electrogram." PACE 1990, 13: 1254-1260. In this method, loss of atrial capture is inferred by the appearance of native A-waves on the AEGM, or atrial sense (AS) and/or atrial refractory sense (AR) markers on the pacemaker marker channel. This is illustrated in FIG. 9 in which the breakthrough of native A-waves can be seen on the AEGM(atrioelectrogram). In this figure the marker channel also provides evidence of atrial senses as AS or AR markers. Note the high degree of atrial non-sensing by the pacemaker due to atrial blanking periods. Native A-waves are marked in FIG. 9 with an up arrow.
Additionally, it should be noted that in the clinical setting native sinus heart rates are likely to be elevated by patient anxiety, and high heart rates are common in younger and older persons, and elevated rates over 100 bpm are commonplace in pediatric or elderly patients with congestive heart failure, thus making the provision of additional isoelectric time made available by the invention described below for visualization of LOAC more universally useful.